Error diffusion halftoning method for dark ink and light ink channels, based on a measure of solvent quantity that should be ejected for each pixel, and apparatus that performs the halftoning method

ABSTRACT

An image processing method suitable for a printer unit, includes an error diffusion halftoning process arranged for quantizing and diffusing each pixel of an image including a set of subtractive primary colors (C′, M′, Y′), in an image including a quantized printer image including a set of ink drops (D C , D M , D Y , D c , D m ) of respective ink channels to be printed. The method comprises the step of determining, for each pixel, an input variable value (S) representing a measure of solvent quantity that should be ejected by the printer unit for each pixel, the input variable value being computed on the basis of the value of a corresponding pixel of the image including the set of subtractive primary colors (C′, M′, Y′), and of inputting the determined input variable value (S) in the error diffusion halftoning process together with the values of the corresponding pixel. The invention further relates to the apparatus embodying the method.

TECHNICAL FIELD

The present invention relates to an image processing method andapparatus, in particular to a colour image processing method andapparatus, and, more particularly, to an error diffusion imageprocessing method and apparatus for colour printers arranged for usingdark and light inks.

BACKGROUND ART

As known, images, such as charts, drawings, and pictures, may berepresented as a two-dimensional matrix of picture elements (pixels).

The spatial resolution and intensity level for each pixel are chosen tocorrespond to the particular output device used. For example, typicalcomputer monitors display images at 75 dots per inch (DPI) and have 256levels of intensity for each colour channel. Such monitors use theadditive primary colours, red, green, and blue (RGB), which can becombined to produce millions of colours, e.g. 256³.

It is also known that typical hardcopy output devices, such as inkjetprinters, use subtractive primary colours, cyan, magenta and yellow(CMY), and are limited in the number n of intensity levels they canproduce for each available colour ink. For each pixel or possible dotlocation on the printed medium they can only print n intensity levels,where n<<256. For standard three ink colour printers n=2 (zero or onedot). If the printer introduces light inks, then n=3 (no dots, one lightink dot, or one standard ink dot). Clearly, some conversion method,referred in the art as “halftoning”, must be provided to convert themonitor-based version of the image (n=256 intensity levels per colour),or any other digital version of the input colour image, to the printerversion (n=2 or n=3 levels per colour), while preserving the continuoustone visual perception provided by monitor images.

One major approach to halftoning is referred in the art as “errordiffusion” or “error diffusion halftoning”.

Error diffusion is not a point process, which means that the decisionabout whether or not to print a dot is based not only on the intensityof the corresponding input pixel, but also on what has happened topreviously processed pixels.

From the paper An Adaptive Algorithm for Spatial Greyscale, Vol. 17 (2),pp 75-77 (1976). 1976 Society for Information Display R. Floyd and L.Steinberg, for instance, a general error diffusion method is known.

The known error diffusion method may be represented with the errordiffusion diagram 10 of FIG. 1 wherein Q represents a quantizer, D adiffuser, reference numeral 12 an adder unit, reference E the errordetermined on the basis of the difference between input values I_(m) tothe quantizer Q and output values O from the quantizer Q.

The unit 12 of FIG. 1 adds image pixel data or input values I of acertain location to errors E_(n) diffused on the basis of previouslyprocessed pixels so to produce in output a modified input data I_(m).The modified input data I_(m) enter the quantizer Q whose output Oindicates which colour dots have to be printed at the correspondinglocation. The error or colour difference E between the input modifiedI_(m) and the output O is then passed to the diffuser D whichdistributes it to further processed neighbouring pixels.The diffused errors E_(n) are, in turn, added to the next processedpixel.The error diffusion algorithm terminates when all the image pixels havebeen serially processed.The known error diffusion method, in particular, diffuses each errorinto a set of four surrounding pixels and it has the particular featureof processing each input colour variable (or colour channel)independently, i.e. the decision to print a dot of ink, for instancecyan, depends only on the value of the colour variable C (cyan), both atthe corresponding image pixel and at the previously processed ones. Thesame holds for the other inks and their associated colour variable.That means that the known Floyd and Steinberg error diffusion methodprovides for the same number of input and output variables.In other words, if we are required to print on a device capable of usingk different colour inks, we need to provide the Floyd-Steinbergquantizer Q with exactly k input variables obtained, for example, by acalibration process arranged for obtaining the input values I, i.e. C,M, Y colour values, according to the following general expression:(C,M,Y)=fmap(R,G,B)  (1)

wherein:

C stands for dark cyan,

M for dark magenta,

Y for yellow,

According to the above expression, for each colour, the final inkquantity deposited on the printing medium is fully controlled by thecorresponding input value I, i.e. C, M, Y. It is the responsibility ofthe printer colour calibration system (through, for instance, the colourcalibration table) to provide zooming-up and conversion from the imageRGB representation to the printer input variables representation (C₁,C₂, . . . , C_(k)) wherein C_(i), with i from 1 to k, are the differentcolour values of the input value I.Therefore, beside the proper colour reproduction, the calibrationprocess has the ability (and is in duty) to control, through the kvariables C_(i), the total maximum amount of ink that will be depositedon the printing medium.

This standard halftoning scheme has the advantage of being simple andreliable but it has an important drawback:

it is computationally very resource demanding.

There are two reasons for that:

first the error diffusion process has to be carried out on all the k inkcolours or channels;

second, the necessary interpolations needed to calculate the input valueI, i.e. C₁, C₂, . . . , C_(k) from the calibration table, have to beperformed within a k-dimensions space.

As a matter of fact, let us first consider a printer which can use onlythree different colour inks, namely Cyan C, Magenta M and Yellow Y.

The very first step in the printing process will be to map the RGBcolour of every input pixel into a CMY ink quantity specification,wherein each possible pixel colour must be precisely specified.

In our example, the calibration or mapping is from a 3-dimensional space(the 3D monitor device colour space) to another 3-dimensional space (the3D printer device colour space) as already reported in expression (1).

The mapping function fmap could be given analytically through amathematical expression, but often it is given in guise of a look-uptable (LUT) where the C, M and Y quantities are numerically expressed,for instance, with eight bits each.

To avoid a lookup table with millions of entries, only a subset of the(R, G, B) to (C, M, Y) mapping points is, in general, included and themissing points are linearly interpolated from the tabulatedtransformations.

The process of finding a suitable mapping function fmap is named“printer calibration” and the resulting look-up table, if any, is called“calibration table”. By controlling the ink quantities, the calibrationtable defines the colour produced on paper by a given RGB input image.Different calibration tables must be created for each printer resolutionand for every type of paper. The main reason is that printing substratesor medium (papers) have substantially different behaviours with regardto the maximum quantity of ink their can absorb.

Beyond a paper-specific threshold, hereinafter referred to as thesaturation limit, the excess of ink prevents the fast drying of thepaper and favours the diffusion of the dyes on the paper surface.Besides the obvious wetting effect, an excess of ink reduces thelightness and the sharpness of the printed image or document. Therefore,an important step in the calibration process consists of the definitionof the said saturation limit. Physically, the saturation limit isrelated to the amount of liquid, or solvent, that a printing substratecan absorb and quickly evaporate. It is important to recognise that thesaturation limit is not directly related to amount of dyes or to theoptical density on paper. The distinction may seem somewhat pedantic aslong as there is a one-to-one relation between the ‘on paper’ opticaldensity and the amount of ink solvent. Unfortunately, such a relationdoes not exist in many cases.

Consider now, for instance, a printer which can print with saturated andnon-saturated inks (also called dark and light inks respectively),typically C, M, Y, c (light cyan) and m (light magenta) inks. In thatcase it is wise to distinguish the amount of ink (C, M, Y, c, m) fromthe amount of dye (C′, M′, Y′) which should be deposited on the paper toproduce a given colour.

The calibration process, in this case, defines the amount of dye throughthe relation:(C′,M′,Y′)=f _(map)(R,G,B)  (2)wherein C′, M′, Y′ are input values I which should be deposited on thepaper to produce a given colour. As easily comprehensible for atechnician in the field, for each amount of dye C′, M′ and Y′ one has:C′=C+ρ _(c) cM′=M+ρ _(m) mY′=Y  (3)Whereρ_(c) and ρ_(m) are the optical density of the light cyan and lightmagenta relative to the optical densities of cyan and magenta;Y′ is assumed corresponding to Y as, in general, printers, as known, arenot provided with light yellow.Preferably, typical values are ρ_(c)=ρ_(m)=⅓.

Except for the Y component, the amount of ink is not uniquely defined bythe amount of dye required. In the most general case there is aninfinite number of couples (C, c) which produce the required quantity ofdye C′.

The special cases (C=0, c) and (C, c=0) correspond to the wettest andthe driest combination, for example, of cyan inks needed to obtain thequantity C′ of cyan dye. The same can be said for the magenta colourcomponent.

As a consequence, the specification of the dyes quantities in thecalibration table does not enable to control the effective amount of ink(solvent) transferred to the printing substrate. The simplest solutionto the problem consists in a calibration table which directly specifiesthe amount of inks instead of the amount of dyes.(C,M,Y,c,m)=Plight(C′,M′,Y′)  (4)To that purpose, a special procedure Plight is introduced whichdetermines the ink amounts (C, M, Y, c, m) from the dye amounts (C′, M′,Y′). The latter are calculated from the calibration map fmap, as shownin equation (2).

As already noted, proceeding that way has some important draw backs:

The number of elements in the calibration table is strongly increased.This will be especially true for printers capable of using five or morethan five inks;

Colour interpolations, for non-tabulated RGB→CMYcm mapping, have to beperformed, for example, at least in a 5-dimensions space, thus requiringmuch more computer resources;

The Plight procedure, which determine the actual ink amounts from thedye requirements, can be difficult to set-up. It requires a number ofarbitrary decision to be taken in order to choose among the manypossible mapping (C′M′Y′)-->(C, M, Y, c, m) for each entry in thecalibration table (see equation 4);

In the quantization process, which occurs in the error diffusion andprinting process, immediately after the calibration, more variables mustbe treated. This can be a waste of computing resources, especially whenthe quantization is done with high quality error diffusion algorithms.

The hardware implementations of the printing algorithms on variousprinters which may use a different set of inks is very difficult. As amatter of fact, having to treat (quantize) a different number of colourvariables depending on the number of inks available on the printer canrequire to use different hardware depending on the number of input andoutput to be managed.

It is, therefore, apparent a problem exists of optimising use ofcomputer resources in case of printers which can print with saturatedand non-saturated inks.

From patent publication US_(—)2003/0214676 an image processing methodand apparatus are known that try to reduce both computer resourcesneeded for implementing the error diffusion method and the overlap ofdots.

The known apparatus provides for using one or more colours for modifyingthe threshold level of another colour in the error diffusion halftoningprocess.

According to the known apparatus the different colours in output fromthe error diffusion are used in input to control the error diffusionhalftoning process.

Such a known prior art suffers the above problem of not substantiallyreducing the computer resources needed to the error diffusion halftoningprocess.

Applicant, in general, has noted that known image processing methods donot effectively solve the problems of optimising at the same timecomputer resources needed for implementing the error diffusion processand the quantity of ink used for printing.

DISCLOSURE OF THE INVENTION

The object of present invention is thus to solve the problems outlinedabove.

According to the present invention such an object is achieved by meansof an image processing method and apparatus having the features setforth in the claims that follow.

The present invention also relates to a computer program productloadable in the memory of at least one computer unit and includingsoftware code portions for performing the steps of the method of theinvention when the product is run on at least one computer unit. As usedhere, the reference to such a computer program product is meant asequivalent to the reference to computer readable medium containinginstructions for controlling a system or a device so as to co-ordinateexecution of the method according to the invention.

Reference to “at least one computer unit” is meant to highlight thepossibility for the method of the invention to be carried out in adecentralised manner over a plurality of computer units.

Claims are an integral part of the teaching of the present invention.

According to a preferred embodiment the image processing method providesan error diffusion halftoning process wherein input variables comprisethe values of a set of subtractive primary colours and the value of avariable, called solvent S, representing a measure of the solventquantity per optical value that is added when a drop of ink is placed inthe paper or printing substrate.

The invention presents, as well, a quantizcr QS, which takes as an inputthe four variables C′, M′, Y′, S and decides which dots are to beprinted among the available ones.

According to a further feature of present invention, the imageprocessing method provides to control the paper wetting through ageneral procedure which does not interfere with the calibration processand that never changes the number of colour variables entering in thequantization process, whatever the number of colour inks used by theoutput device. Such a feature is achieved in the preferred embodiment bya two step operation. Firstly by introducing the new variable S, whichwill go through the quantization process as a normal colour component.Secondly, by providing the new quantizer QS, that will include an entryfor each C′, M′ and Y′ dye quantities as well as for the solventvariable S. The quantizer QS will have an output for each ink used inthe actual printing device and will be designed in such way to choosethe output depending not only on the colour requirement C′, M′ and Y′but on the solvent S as well.

BRIEF DESCRIPTION OF DRAWINGS

These and further features and advantages of the present invention willappear more clearly from the following detailed description of apreferred embodiment, provided by way of a non-limiting example withreference to the attached drawings, in which components designated bysame or similar reference numerals or characters indicate componentshaving same or similar functionality and construction and wherein:

FIG. 1 shows a general block diagram for an error diffusion methodaccording to prior art;

FIG. 2 shows a block diagram for an error diffusion method according tothe invention;

FIG. 3 represents possible solvent quantities S that are allowed tobuild a given colour;

FIG. 4 a represents an example of method for calculating solventquantities S;

FIG. 4 b represents an example of solvent calculation from RGB;

FIG. 5 represents a graphical example of a quantizer arranged for beingused in the error diffusion method of FIG. 2; and

FIG. 6 shows a block diagram of an image processing method including theerror diffusion method of FIG. 2;

BEST MODE FOR CARRYING OUT THE INVENTION

With reference to FIG. 2 an image processing method for colour printers,according to a preferred embodiment of present invention, is representedas a block diagram comprising an error diffusion diagram 110 arrangedfor receiving as input the output of a conventional calibration processC, i.c. the values C′, M′, Y′ already discussed, and a solvent variableS the value of which should be viewed as the measure of the quantity ofsolvent that should be placed on the paper.

According to the preferred embodiment of present invention the solventvariable S is determined by a solvent calculation block SC on the basisof values C′, M′, Y′ as will be disclosed later on in detail.

The values of C′, M′, Y′ and S are, according to the preferredembodiment of present invention, the set of input values I to the errordiffusion diagram 110.

The error diffusion diagram 110 comprises a quantizer QS, as will bedisclosed later on in detail, arranged for receiving as input a set ofinput values I_(m), for instance a set of four values from an adder unit12 of known type, and generating in output a set of output values O in anumber corresponding to the number of colours or ink channels managed bythe printer; for instance, according to the present preferred embodimentthe set of output values O comprises five possible values D_(C), D_(M),D_(Y), D_(c), D_(m) indicating presence or absence of a dot ink to beejected.

In the framework of present invention the expression ink channel is usedfor indicating inks of different colour or different saturation degreethat are available to the printer unit.

The error diffusion diagram 110 further comprises an error block ES, aswill be disclosed later on in detail, arranged for determining, on thebasis of the set of output values O and of the set of input values I_(m)an error E to be diffused by means of a diffuser D, of known type.

The diffuser D is arranged for supplying, in known way, a set of errorvalues E_(n) to the adder unit 12 for being added to the set of inputvalues I, i.e. to the values C′, M′, Y′, and S.

Solvent Computation

According to the preferred embodiment the computation of the solventquantity S is performed independently for each colour channel C′, M′, Y′and the final quantity S is the addition of the three obtainedquantities.

Preferably, the computation of the solvent quantity S should try tomaximise the usage of light inks without reaching the saturation limit.It is important as well to provide a smooth transition between thecolours where light inks are predominantly used and those produced withdark inks only.

As will be apparent to a technician in the field, further methods may beused for determining the solvent quantity S wherein it could be anysuitable function of the colour variables, without departing from thescope of the invention as claimed.

According to the preferred embodiment the solvent computation supposesit is optimal to use only light inks if the value to represent issmaller than a given threshold T. On the contrary, if the value torepresent is bigger than this threshold, it will use more and more darkinks, replacing light dots by dark dots in the printing.

So, let T_(X) be a threshold on C′ and M′, and X be the value of the C′or M′ channels or colours as shown in FIGS. 3 and 4 a, then thecomputation of the solvent is performed as shown in the followingexpression (5):

IF X<T_(x)S _(X)=α_(X) X  (5)ELSES _(X)α_(X) T _(x)+β_(X)(X−T _(X));

wherein:

X is a colour intensity;

T_(X) is a certain threshold;

S_(X) is a solvent value corresponding to a certain value of X;

X_(max) is the maximum colour intensity;

S_(max) is the maximum solvent value for a certain paper;

S_(x) values contained in the shaded triangular area of FIG. 4 a areless or equal of S_(max) values of FIG. 3;

α_(x) is the solvent quantity per optical density that a light drop ofthe X dye brings; and

β_(x) is a percentage value obtainable as(S_(max)−α_(X)T_(X))/(X_(max)−T_(X))

As would be clearly apparent to a technician in the field, according tofurther embodiments, the computation of the solvent quantity may change,according to the application, the sort of inks or other factors, such asthe required visual quality and/or ink consumption.

The constants α_(X) and β_(X) may depend on the channel, in order totake into account possible differences among the optical densities ofcolour inks.

According to the preferred embodiment, the expression (5) is only usedto compute the solvent quantity of the cyan and magenta channels.

This is due to the fact that the yellow does not normally have, asalready discussed, a light ink; however the solvent quantity of theyellow channel has to be taken into account in order to compute thesolvent and the saturation value of the paper.

The computation of the yellow solvent quantity S_(Y′), isstraightforward:S_(Y′)=β_(Y′)Y′  (6)

wherein, for example, β_(Y′) is equal to 1.

Finally, the total solvent quantity that is placed on a paper sheet isthe addition of the solvent quantities of the three different inks:S=S _(C′) +S _(M′) +S _(Y′)  (7)

It should be clear that the solvent quantity S, preferably, has to becalculated from the dyes quantities C′, M′, Y′ obtained from thecalibration process described by equation (2) and shown on FIG. 2.

According to the preferred embodiment, the ink quantities C, M, Y, c, mare not known, and neither required, since their quantities areimplicitly provided by the variable S(C′, M′, Y′).

In order to better clarify solvent S (S_(tot)) calculation starting fromRGB, in FIG. 4 b a reduced set of RGB values is presented. For each RGBthe corresponding C′, M′, Y′ shows he typical transformation provided bya printer calibration table. Then, for each color component, the solventS_(C′), S_(M′), S_(Y′) is calculated according to expression (5) and(6). Finally, the total solvent component S_(tot) is calculated usingexpression 7.According to the example, the parameters used in the calculation are:α_(C′)=α_(M′)=3β_(C′)=β_(M′)=0.434 which correspond T _(C′) =T _(M′)=30; andthe yellow component which do not have a light ink componentβ_(Y′)=1 and T _(Y′)=0.The shaded columns form the final calibration table with solvent S.

Error Diffusion

As known, the error diffusion process is not a point process. It takesthe decision of whether or not to print a dot according to what hashappened to previously processed pixels, in such a way that the localaverage colour density matches the required density, wherein therequired colour density is defined by the input image and thecalibration function fmap; in other words, error diffusion allows tolocally fulfil the colour input requirements on average, not on onesingle pixel.

According to the preferred embodiment of present invention the variableS is included into the error diffusion process. Once the variable S iscalculated, for example as already disclosed, all we need is a quantizerQS which takes the modified inputI _(m)(C′ _(m) ,W′ _(m) ,Y′ _(m) ,S _(m))  (8)wherein C′_(m), M′_(m), Y′_(m) are theoretical colour values to beprinted;S_(m) is a theoretical solvent value to be used;and produce the output:O=(D _(C) ,D _(M) ,D _(Y) ,D _(C) ,D _(m))  (9)wherein D_(C), D_(M), D_(Y), D_(c), D_(m) (D_(X)) are real combinationcombinations of drops of ink to be printed or not.

As a matter of fact, the output O specifics which ink drops have to beplaced at the corresponding pixel location. For instance, if a drop ofink X, corresponding to any one of colours, has to be printed, thenD_(X)=1, otherwise D_(x)=0.

The error block ES is arranged:

first of all, for associating to each combination of ink drops, forexample on the basis of a Look-Up Table (LUT), a value V_(out) accordingto the following expression wherein a dye and solvent value isassociated to each possible output O:V _(out)(O)=(C′ _(o) ,M′ _(o) ,Y′ _(o) ,S _(o))  (10)

secondly for calculating the error E, at a given pixel, between themodified input values I_(m) and the output value V_(out) by using, forexample, the following expression:E _(X) =I ^(X) _(m) −V ^(x) _(out)  (11)wherein:X represents the colour dyes or the solvent (C′, M′, Y′, S); andE_(X) represents each component of the error E.

The error is then diffused in a known way by the diffuser D, and addedto further processed pixel value. According to the preferred embodiment,the above error diffusion method has the following features which areworthy to be emphasized:

First of all, the number of input variables is, preferably, a fixednumber independent of the number of output variables;

Secondly, as it should be already clear, its input variables are notrelated to the ink quantities, but represents the amount of dye neededto produce a required colour as obtained by a conventional calibrationprocess, as described by equation (2) and shown on FIG. 2, with theaddition of a fourth variable which controls the amount of solvent usedas a dyes carrier;

Thirdly, the forth variable is, preferably, not independent of the firstthree. The solvent S is a function S=S(C′, M′, Y′) of the dye variables.

The last item, in particular, has to be considered with care because itmay be used for granting a further feature related to error diffusionstability problem.

As known, the whole error diffusion process works only if the inputrequirements can be met on a local average.

If the input I contains some impossible combination of values, thereported error will become exceedingly large and the reproduced imagewill, therefore, diverge from the input image. Unpredictable and verymuch distorted output images can be expected.

Consider the case of a constant input I having a constant componentI^(X) and assume that it is larger than any available output value V^(X)_(out). The error E_(X) will necessarily be positive and thereforeincreases the modified input I^(X) _(m) value for the next processedpixel. Again, the following error E_(X) will be necessarily positive andwill clearly continue to grow for each subsequently processed pixel.

A different but equivalent view of the stability problem may be viewedby means of FIG. 3 which represents the possible solvent quantities thatare allowed to build a given colour.

The solvent quantity S that will be used to build a given colour ishigher if light inks are used, and lower if we use dark inks. Thegraphic shows the possible solvent quantities that can be placed on apaper to build a given colour. Of course, there are solvent quantitiesthat are impossible, for example, one cannot build a very dark colourwith less solvent than its input value. If the colour is entirely builtwith light inks, the solvent quantity increases with a slope bigger thanone with respect to the colour channel intensity value. Instead, if thecolour is built entirely with dark inks, the quantity of solvent to beplaced on the paper is conventionally considered to be exactly theintensity of the colour channel (considering that placing a dot of dyeon the paper is equivalent to placing a conventional value 255 ofsolvent). The computation of the solvent quantity needed to represent agiven colour is then something in between, as already disclosed, closerto the light ink or dark ink curve depending on the designer preferencesto favour the use of light or dark inks. As explained above, the centraltriangular area corresponds to all the combinations of solvent (S) anddye (Colour Channel intensity) that can be reached by combining dark(standard or saturated) and light (non-saturated) inks. The trianglecorners are the three possible output value V_(out) in a single colourplane. For instance, in the cyan plane, we have white, light cyan anddark cyan:V _(out)(white)=(0,0,0,0)V _(out)(light cyan)=(80,0,0,255)V _(out)(cyan)=(255,0,0,255)  (12)

According to the example:

for the system to be stable in the plane (C′, S), the input valuesI_(C′) must be within the triangular area bounded by the three points ofequations 12;

equivalent conditions hold for stability in the plane (M′, S); and

in the plane (Y′, S) the system is stable only if S=Y′ is strictlysatisfied.

According to the preferred embodiment of present invention the inputvalues I are controlled in order that they stay into the possible rangesalready defined in expression (12), i.e. the maximum values provided forV_(out); such values represent the needed stability conditions for thequantizer QS.

Moreover, because in the error diffusion process the input to thequantizer unit is modified by the addition of the errors from thepreviously processed pixels and could be that the modified input is nomore in the interval [0, 255], in our preferred embodiment, thequantizer is only defined on this range of values [0, 255], bytruncating the out of range values of input I_(m) according to thefollowing simple rule:IF X<0X=0ELSE IF X>255  (13)X=255ELSEX=Xwherein:X represents the colour dyes or the solvent (C′, M′, Y′, S).Solvent Adaptive Quantizer

The error diffusion process of the present invention comprises,according to the diagram 110 (FIG. 2), a quantizer process (quantizer)QS arranged for managing, further to the variables C′, M′ and Y′,derived by the calibration process, the disclosed variable S, which is,obviously, larger for light inks than for dark or standard inks.Therefore, according to the preferred embodiment of present inventionthe quantizer QS is arranged to favour the output of light inks when Sis large and the output of standard inks when S is small.

In the following, we present an implementation example of a quantizerthat satisfies the above conditions.

We consider a quantizer QS that takes the four modified input variablesof equation (8) and generates the five output channels of equation (9).

This quantizer QS is represented graphically in FIG. 5 and could bethought as a 4-D quantizer, where the fourth dimension is the solventvalue.

In order to facilitate the visualisation of such a quantizer, we willconsider from now on a set of different 3-D quantizers whereof FIG. 5shows the C′, M′ representation as a function of S.

In the graphical example of FIG. 5 it is assumed that the value of S(S_(index)) is the input to a look up table wherein on the basis of thevalues of C′_(m) and M′_(m) the output value O is obtained.

For instance, if we have reached a point where the allowed solventquantity of a given pixel is very low, we will point to a quantizerwhere the use of light inks is not allowed and all the colours areperformed exclusively with standard inks (FIG. 5, representation withS_(index)=1).On the contrary, if the solvent quantity of a given pixel is very high,the quantizer will prioritise the use of light inks, thus, will have awide zone of light inks on it, and smaller dark inks zones (FIG. 5,representation with S_(index)=16).

FIG. 5 is also useful for showing how the surface ratio, of the areasthat output dark and light inks, changes with the solvent value andshould help to understand how the solvent variable S influences thedeposition of light and dark ink drops.

As easily comprehensible for a technician in the field, the quantizer QSas disclosed is only an example of possible quantizers.

As a matter of fact, any quantizer arranged for evolving with thesolvent quantity could be used without departing from the scope of theinvention as claimed, provided it will fulfil the above requirements,i.e. to favour the output of light inks when S is large and the outputof standard inks when S is small.

An apparatus arranged for applying the image processing method asdisclosed comprises a controller board or unit for controlling a printerunit arranged for using, for instance dark and light inks.

Preferably, the controller board comprises a microprocessor arranged forstoring a set of computer program modules realised during the designphase of the printing unit for implementing the image processing methodas disclosed.

According to further embodiments the set of program modules may bestored partially in the controller board and partially in a computersystems connected in known way to the image processing apparatus.

According to the preferred embodiment of present invention the set ofprogram modules comprises:

one or more calibration modules, of known type, arranged for convertingadditive colours into subtractive colours by means, for example, of aLook Up Table and Interpolation;

one or more modules for calculating, on the basis of the subtractivecolours, a variable S suitable for supplying a measure of the quantityof solvent that should be placed on the printing medium;

-   -   one or more modules arranged for operating an error diffusion        halftoning process having a limited number of inputs, for        example less than or equal to four inputs including the        subtractive colours and the variable S, and a number of outputs        depending on the printing unit, preferably greater than the        number of inputs.

The operation of such an apparatus is as follows.

By keeping as reference the preferred embodiment wherein it is provideda printing unit arranged for printing by using dark and light Cyan (Cand c), dark and light Magenta (M and m), dark Yellow (Y):

in a first step 210 (FIG. 6), following a START step, a calibrationprocess is provided for zooming-up and converting each pixel of an imageRGB representation to the C′, M′, Y′ representation;

in a second step 220 the values of C′, M′, Y′ are used for determiningthe solvent value S; according to the preferred embodiment of presentinvention such a value is, by definition, comprised in a range of valueswhich does dot exceed the maximum allowed value for a certain printingsubstrate or medium;

-   -   in a third step 230, following a step 225 arranged for inputting        to the error diffusion process the values C′, M′, Y′, S as input        values I, the error diffusion halftoning process is arranged for        quantizing and diffusing the input information I and outputting        the information to be printed O=(D_(C), D_(M), D_(Y), D_(c),        D_(m)); preferably the quantizer QS is arranged for managing the        solvent variable or value S as an input value to a look-up table        in order to provide an output such that the quantity of ink does        not exceed the maximum allowed value for a certain paper; more        preferably the input values I_(m) to the quantizer QS are        limited in a range of allowed values, for instance values        conventionally comprised between 0 to 255.

The operation of the apparatus is ended by outputting as END step thequantized image.

Advantageously, the image processing method, as disclosed, provides forusing a predetermined limited number of input variables I to the errordiffusion process.

Moreover, the input variables I are independent of the number of outputsfrom the error diffusion process. The limitation of the input variablesprovides that computer resources needed for the error diffusion processare limited both in terms of computation time and in terms ofcomputation devices to be used. In particular, by limiting the number ofinput variables, for example to a predetermined number, it is possibleto standardisc the type of computation means and to use standardisedhardware for the error diffusion process.

Advantageously, the image processing method, as disclosed, provides forusing a new variable named solvent S, which, according to the preferredembodiment, is intended as a measure of the solvent quantity per opticalvalue that is added when a drop of ink is placed in the printingsubstrate. Use of the solvent variable S grants that the imageprocessing method is intrinsically apt to control the paper wetting,i.e. the quantity of ink used for printing.

Of course, obvious changes and/or variations to the above disclosure arepossible, as regards shapes, materials, components, circuit elements andconnections, as well as details of circuitry, of the describedconstruction and operation method without departing from the scope ofthe invention as defined by the claims that follow.

The invention claimed is:
 1. An image processing method implemented in acontroller unit adapted to control a printer unit, the controller unitadapted to perform an error diffusion half-toning process for quantizingand error diffusing each pixel of an image, wherein the image includes aquantized printer image (O) including a set of ink drops (D_(C), D_(M),D_(Y), D_(c), D_(m)) of respective ink channels that are a realcombination of drops of ink to be printed, the method comprising:determining for each pixel, by the controller unit, a set of dyequantities cyan, magenta, and yellow (C′, M′, Y′) of subtractive primarycolours and a further colour input variable value (S) representing ameasure of solvent quantity that should be ejected by the printer unitfor each set of dye quantities, the further colour input variable value(S) being computed by calculating for each dye quantity of the set ofdye quantities (C′, M′, Y′) a respective measure of solvent quantity(Sx), by limiting said respective measure of solvent quantity (Sx) to beless or equal to a maximum measure of solvent (Smax) for a certain paperand by obtaining as a final colour input variable value (S) the additionof the respective measures of the solvent quantities (S_(x)); providing,by the controller unit, the determined further colour input variablevalue (S) and the set of dye quantities (C′, M′, Y′) as a limited numberof colour input variables (I) to the same error diffusion half-toningprocess; adding error output values (E) to said limited number of inputvariables in the error diffusion process to generate values of a limitednumber of modified input variables (Im); quantizing (QS), by thecontroller unit within said error diffusion half-toning process, saidlimited number of modified input variables (Im), said further colour(s_(index)) being used as an input for a look up table whereon on thebasis of the values of the modified input variables (C′_(m), M′_(m)) aquantized printer image (O) including a set of ink drops (D_(C), D_(M),D_(Y), D_(c), D_(m)) of respective ink channels used in the actualprinting device is obtained.
 2. The image processing method according toclaim 1, wherein said error diffusion half-toning process furthercomprises: converting the quantized printer image (O) into output values(V_(out)(O)) by using a look-up table arranged to associate eachcombination of ink drops of said quantized printer image (O) to a set ofdye quantities (C′_(o), M′_(o), Y′_(o)) and a further color inputvariable (S_(o)) representing a measure of solvent; generating erroroutput values (E) having said limited number of colour input variables(I) by comparing respectively said output values (Vout(O)) and thevalues of the limited number of modified input variables (Im).
 3. Theimage processing method according to claim 1, further comprising:receiving the image including a set of values of additive primarycolours Red, Green, Blue (R, G, B) for each pixel and converting it toan image including the set of dye quantities (C′, M′, Y′) of subtractiveprimary colours for each pixel.
 4. The image processing method accordingto claim 1, wherein said error diffusion half-toning process comprisesthe step of: outputting a quantized printer image including dots of darkand light inks (D_(C), D_(M), D_(Y), D_(c), D_(m)).
 5. The imageprocessing method according to claim 1, wherein said quantizer process(QS) further comprises: truncating the modified input values into arange of admissible values for a colour input variable value (Sm) andfor values of a set of dye quantities (C_(m)′, M_(m)′, Y_(m)′) ofsubtractive primary colours.
 6. The image processing method according toclaim 1, wherein said set of dye quantities (C′, M′, Y′) of subtractiveprimary colours and said further colour input variable (S) determinesaid limited number of colour input variables (I) which is independentof the number of ink channels (D_(C), D_(M), D_(Y), D_(c), D_(m)) to beprinted.
 7. The image processing method according to claim 6, whereinsaid limited number of colour input variables (I) is less than or equalto
 4. 8. A controller unit for a printer unit, comprising a controllerunit connectable to the printer unit and adapted to control ink ejectionby the printer unit, said controller unit comprising a microprocessorstoring program modules adapted to perform the method of claim
 1. 9. Anon-transitory computer readable medium including software code portionsarranged to perform, when run on at least one computer, the methodaccording to claim 1.